The digital synthesis of a tone of a specified frequency is described in a paper entitled "A Digital Frequency Synthesizer" by J. Tierney, C. M. Rader and B. Gold which appeared in the IEEE Transactions on Audio Electroacoustics, vol. AU-19, at pages 48-56, in March 1971. The technique described therein utilizes digital techniques followed by analog filtering to produce an analog tone of a prescribed frequency.
It has been found useful to synthesize sounds composed of a plurality of tones for training people to recognize specific sounds. Such sounds may be recognized auditorily as by playing the sounds through a loud speaker, or visually as by passing the sound through a spectrum analyazer and displaying the spectrum to produce a visually identifiable signature of the sound spectrum. For example, such a sound synthesizer may be utilized in the training of automotive mechanics to recognize specific sounds in an automobile engine, which sounds may be masked by other sounds or noise of the engine. Recognition of the specific sounds would be useful in identifying the presence of a specific malfunction in the engine.
A problem has arisen in the past when a sound synthesizer has been utilized with a visible display of the spectral signature due to the fact that a spectral analysis of a naturally occurring sound, such as the sound of an automobile engine, produces relatively broad spectral lines at various ones of the tone frequencies present in the sound. In contrast, the sounds produced by synthesizers of the prior art and composed of a plurality of tone frequencies result in a spectral signature showing relatively sharp spectral lines at the respective tone frequencies. The difference in line width makes recognition of the spectral signature more difficult and decreases the utility of the sound synthesis technique as a means for teaching the identification of specific sound patterns.